A microbolometer is the latest type of thermal imaging Focal Point Array (FPA), which consists of materials that measure heat by changing resistance at each pixel. A microbolometer is a type of infrared detector, which absorbs the infrared (IR) radiation and warms slightly. The electrical resistance across the bolometer changes as a function of temperature, which can be measured and calibrated. The most common microbolometer material is vanadium oxide (VOx). Amorphous silicon (a-Si) is another microbolometer material. The a-Si model has poor dynamic range and isothermal scene performance, which limits the current version(s) for many fire service applications.
It has been the practice in a bolometer type infrared sensor to use a titanium oxide film, a vanadium oxide film or a similar film. Examples are described in U.S. Pat. No. 5,286,976 issued to Barret E. Cole, U.S. Pat. No. 5,801,383 issued to Hideo Wada, an article contributed by Hubert Jerominek and others to Optical Engineering, v.32 (1993) n.9, pages 2092 to 2099 under the title of “Vanadium Oxide Films for Optical Switching and Detection,” an article contributed by D. P. Partlow and others and to Journal of Applied Physics, v.70 (1991) n.1, pages 443 to 452, under the title “Switchable Vanadium Oxide Films by a Sol-Gel Process.” Partlow discloses that vanadium oxide has thirteen distinct phases between vanadium dioxide and vanadium sesquioxide (rendering this material not stable enough).
Titanium oxide has a specific resistance as low as 0.01 Ohm.cm, however, the temperature coefficient is not so great in absolute value, being −0.2% per degree Celsius. On the other hand the specific resistance of vanadium dioxide is about 10 Ohm-cm when manufactured by sputtering. Its temperature coefficient is about −2%, when manufactured without additional temperature treatment. When additional thermal treatment is included, the TCR can be increased to 4% (see, for example, U.S. Pat. No. '383 to Wada). However the treatment temperature is more than 350° C., which is not sufficiently compatible with the very large scale integrated circuit (VLSI) manufacturing process of the whole infrared sensor. Moreover, as described in U.S. Pat. No. '383, to Wada, the vanadium dioxide is susceptible to a metal-semiconductor phase transition at about 70° C. This gives rise to a volume variation of vanadium dioxide and causes cracks and peel off to render a vanadium oxide film unreliable when it is used in the bolometer-type infrared sensor device, which is unavoidably subjected to temperature cycles passing through 70° C.
In U.S. Pat. No. 6,512,229 to Saski, et al, a process for preparing the crystal phase of V2O3, with x=1.5 in VOx is disclosed. A TCR of about 2-4% is obtained after applying a heat treatment of 380° C. for 8 hours in hydrogen. However, the resistivity of these films is too low, about 10−3-10−4 Ohm-cm. To tailor the resistivity to the needed values, about 10−1 Ohm-cm, an additional oxidation heat treatment was applied.
Thus, it would be advantageous to provide a microbolometer film material with a high value of thermal coefficient of resistance, fabricated at temperatures compatible with CMOS technology.